LONDON - J.K. Rowling looked slightly tense. She had just completed a television interview, alongside her collaborators on “Harry Potter and the Cursed Child,” the eighth — and the only theatrical — installment in her celebrated wizarding saga, which begins previews on March 16 and opens on Broadway on April 22.

Her Harry Potter novels have sold more than 500 million copies worldwide, spawning an empire that encompasses movies, the spin­off “Fantastic Beasts” films, Harry Potter Wizarding World theme parks and detective novels (written under a pseudonym) that have been made into a television series.

Like much that Rowling touches, her first theatrical venture has been a smash. It earned rave reviews and sold-out houses in London, going on to win in a record-breaking nine categories at last year’s Olivier Awards, the British equivalent of the Tonys.

Yet, she is determined to take absolutely nothing for granted.

“We see this as a new challenge,” she said, looking at John Tiffany, the show’s director, and Jack Thorne, its writer, who were seated with her backstage at the Palace Theater. “Broadway is a scary place.”

Secrecy about the story line, collectively developed by Rowling, Thorne and Tiffany, fed a growing obsession with what the play would reveal about Harry and Company, and a #keepthesecret campaign encouraged a clublike camaraderie among the preview audiences.

But now that script has been published, reviews have been written and tweets have been tweeted, the plot — a coming-of-age trajectory for Harry’s second son, Albus — is out there.

Still, there are other questions awaiting answers.

No music

Unlike most family-oriented Broadway offerings, “Cursed Child” is a play, not a musical, and it will face stiff competition for the family audience from Disney’s musical adaptation of the animated blockbuster “Frozen,” which is opening March 22.

“It is unusual to take such a large brand franchise and not musicalize it,” said Sonia Friedman, who, with Colin Callender, has produced the play in London and New York. “In every place possible, we say, ‘A new play by.’ ”

One other thing: It has a running time of five hours 15 minutes, and is staged in two parts (either seen on one day or on different nights).

Asked whether they had encountered any resistance from parents to a five-hour-plus commitment, Rowling gave a firm no. For kids under 9, she said, “it might be stretching it.”

Friedman pointed out that the majority of the fan base is the generation that grew up reading Harry Potter and now is age 25 to 35.

“We learned a lot from the London show about the community feeling that happens when people watch two parts in succession,” Friedman said. “Part of the experience is what happens at the intervals, between the shows.”

Tiffany added, “When we thought of the nature of the story and talked about what became the end of Part 1, we just couldn’t get there in an hour.”

Callender said they had realized it was important that viewers keep the same seats for both parts of the play and recognize their seatmates. “Then you are on a journey together,” he said.

Shared creativity

Rowling has maintained a tight lock on the Potterverse empire. Her willingness to put her characters in other hands is surprising, but she said she loved the entire process of co-creating “Cursed Child.”

Although the play will essentially remain the same on Broadway as it was in London, the creative team is not taking anything for granted.

“If we see audiences aren’t getting anything, we’ll obviously adjust,” Tiffany said. “We’re always working on the show, and there are certain things about the architecture of the Lyric which means some things will change. It’s a theater with different kinds of possibilities, and I want to exploit them all.”

Rowling laughed. “Never leave a possibility unexploited,” she said.

Asked if she ever worried about her fans’ reactions to extending a universe they treasure, Rowling sat up straighter.

“I think,” she said calmly, “that it’s up to me what I do with the world I created.”

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